From Sahih Muslim (Hadith 2909):
Jabir b. `Abdullah (Allah be pleased with them) reported that when Asma' bint `Umais gave birth (to a child) in Dhu'l-Hulaifa. Allah's Messenger (ﷺ) commanded Abu Bakr (to convey to her) that she should take a bath and enter into the state of Ihram
Reference: Muslim 2909